Monoclonal antibodies (mAbs) are successful as therapeutics due in part to their ability to bring to bear the destructive capabilities of the immune system against specific target cells. In a variety of in vivo and in vitro settings, antibody coating of targets has been shown to mediate potent killing mechanisms such as complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), and antibody-dependent cellular phagocytosis (ADCP). All of these effector functions are mediated by the antibody Fc region.
The “classical” complement system is the antibody-dependent complement cascade, which consists of over twenty tightly-regulated proteins, C1 through C9. The trigger for classical complement activation is the initial binding to antibody-coated target by complement protein C1q, a bundle of six heterotrimeric subunits composed of globular heads and collagen-like tails.
The most widely recognized mechanism of complement-mediated target destruction is lysis by the membrane-attack complex (MAC), a transmembrane channel created by complexation of C5b, C6, C7, C8, and C9 proteins. This noncellular process, commonly referred to as CDC, is thought to be relevant to the clinical activity of some anti-tumor antibodies (Gelderman et al., 2004, Trends Immunol 25[3]:158-64). Less established for antibody drugs, although potentially no less relevant, are cellular-based complement mechanisms that are mediated by interaction between opsonic C3 and C4 components and complement receptors (CR1, CR3, and CR4) expressed on effector cells.
Optimization of complement activity appears as an unlikely means to increase therapuetic antibody efficacy. Take for example the therapuetic anti-CD20 antibody rituximab. Complement protein C3 has been shown to inhibit rituximab-mediated natural killer (NK) cell activation and ADCC (Wang et al., 2008, Blood 111[3]:1456-63), impacting antibody activity in vivo (Wang et al., 2009, Blood). Further, there is the observed absence of complement-mediated cytotoxicity in vitro using tumor cells from different response groups (Weng et al., 2001, Blood 98[5]:1352-7), and the uncompromised activity of other anti-CD20 mAbs in complement deficient mice (Hamaguchi et al., 2005, J Immunol 174[7]:4389-99; Uchida et al., 2004, J Exp Med 199[12]:1659-69). Thus, taken in whole these reports cast doubt on increasing the efficacy of therapuetic antibodies by altering complement activity.
In spite of this, the instant disclosure provides variant polypeptides, Fc, that alter complement activity. In the context of anti-CD20 antibodies, these Fc variants demonstrate increased efficacy relative to anti-CD20 antibodies without these Fc variants.